全文获取类型
收费全文 | 682篇 |
免费 | 65篇 |
国内免费 | 39篇 |
专业分类
化学 | 399篇 |
晶体学 | 5篇 |
力学 | 16篇 |
数学 | 24篇 |
物理学 | 238篇 |
无线电 | 104篇 |
出版年
2023年 | 7篇 |
2022年 | 6篇 |
2021年 | 18篇 |
2020年 | 12篇 |
2019年 | 16篇 |
2018年 | 27篇 |
2017年 | 16篇 |
2016年 | 17篇 |
2015年 | 32篇 |
2014年 | 32篇 |
2013年 | 47篇 |
2012年 | 69篇 |
2011年 | 70篇 |
2010年 | 44篇 |
2009年 | 44篇 |
2008年 | 40篇 |
2007年 | 30篇 |
2006年 | 21篇 |
2005年 | 24篇 |
2004年 | 22篇 |
2003年 | 15篇 |
2002年 | 13篇 |
2001年 | 17篇 |
2000年 | 14篇 |
1999年 | 13篇 |
1998年 | 11篇 |
1997年 | 11篇 |
1996年 | 11篇 |
1995年 | 7篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 6篇 |
1989年 | 3篇 |
1988年 | 5篇 |
1987年 | 8篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1975年 | 3篇 |
1974年 | 7篇 |
1973年 | 5篇 |
1972年 | 2篇 |
1970年 | 2篇 |
1968年 | 1篇 |
1967年 | 5篇 |
排序方式: 共有786条查询结果,搜索用时 15 毫秒
21.
A. Concheso R. Santamaría R. Menéndez J. M. Jiménez-Mateos R. Alcántara P. Lavela J. L. Tirado 《Journal of Solid State Electrochemistry》2005,9(9):627-633
The effect of oxidative stabilization as a mean to modify the carbon texture was essayed in a group of mesophases previous to carbonization at 900 °C with the aim of evaluating the influence on electrochemical performance when used as electrode materials in lithium test cells. X-ray diffraction, optical microscopy and chemical analysis, Fourier-transform infrared spectroscopy have been used to describe the compositional and textural properties of the as-produced parent mesophases, the samples were further treated under air current to stabilize their microstructures and the corresponding carbonized samples at 900 °C. The electrochemical performance was determined by the galvanostatic method and further correlated to the physical–chemical properties and interface resistance of the materials. In all cases, the stabilization process has demonstrated a beneficial effect on the capacity retention in the measured range. 相似文献
22.
表面修饰二氧化锡纳米微晶的制备与表征 总被引:10,自引:0,他引:10
制备了硅烷偶联剂KH-570表面修饰的SnO2纳米微晶,通过FT-IR、XPS、TEM和TG-DTA对其结构和表面特性进行表征和研究. FT-IR和XPS分析结果确证了KH-570与SnO2表面是以化学键合或物理吸附方式相结合,粒子表面存在酯基等有机官能团的红外吸收特征;观测到KH-570中Si原子的Si2s和Si2p谱线. TEM分析表明,表面修饰反应增强了SnO2纳米微晶的疏水性和分散性.由XPS和TG的实测数据探讨了纳米粒子具有较低包覆量的可能原因. 相似文献
23.
A. Yu. Men’shikova B. M. Shabsel’s T. G. Evseeva N. N. Shevchenko A. Yu. Bilibin 《Russian Journal of Applied Chemistry》2005,78(1):159-165
The capability of carboxylated latexes of poly(methyl methacrylate) and copolymers of styrene with glycidyl methacrylate or methacrylic acid for self-organization in thin films was studied in relation to the compositions of the dispersion medium and polymer and to the latex particle size.__________Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 1, 2005, pp. 161–167.Original Russian Text Copyright © 2005 by Men’shikova, Shabsel’s, Evseeva, Shevchenko, Bilibin. 相似文献
24.
Ana I. Olives Emiliano E. Romero-Ale M. Antonia Martín Benito del Castillo Pilar López-Alvarado J. Carlos Menéndez 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):553-559
Carbazole-9-carbonyl chloride (C9CC) and 9-carbazolylacetic acid (9CAA) were selected as model fluorescent reagents. The effect
of different chemically modified cyclodextrins (CDs) added to the aqueous solutions of these reagents was studied in water
and in buffered aqueous solutions at pH 4.5 and 8.8. The CDs employed were 2-hydroxypropyl-β-cyclodextrin (HP-βCD), 2,3-di-O-methyl-β-cyclodextrin (DM-βCD) and 2,3,6-tri-O-methyl-β-cyclodextrin (TM-βCD). The inclusion of these reagents inside the cavities of the CDs was verified and this process
can affect the derivatization reaction because CDs can modify the reactivity of the guest molecules. The basic conditions
necessary for the derivatization reaction between C9CC and amines lead to the formation of carbazole anion through hydrolysis
followed by decarboxylation. In the presence of CDs, the hydrolysis-decarboxylation of carbazole-9-carbonyl chloride is faster
than in buffered aqueous homogeneous solutions. The behaviour observed for these reagents in aqueous solutions of CDs was
compared to the one observed in basic ethanolic solutions. These changes are particularly noticeable in the case of 2,3-di-O-methyl-β-CD and 2-hydroxypropyl-β-CD. The characteristics of the fluorescent reagents are compared to carbazole and 9-methylcarbazole
as model compounds.
This paper was presented at XIIIth International Cyclodextrin Symposium. Torino, Italy, May 14–17, 2006. 相似文献
25.
Hevia E Pérez J Riera V Miguel D Campomanes P Menéndez MI Sordo TL García-Granda S 《Journal of the American Chemical Society》2003,125(13):3706-3707
As depicted in the scheme, the alkylidenamido complex 1, a N-rhenaimine, reacts with ketenes to afford the beta-lactams 2-4, which possess a {Re(CO)3(bpy)} fragment as substituent at nitrogen. Clean demetalations using HOTf or MeOTf yield the free beta-lactams or N-methyl-beta-lactams along with [Re(OTf)(CO)3(bpy)]. DFT calculations help to rationalize why the reaction is faster than those of non transition metal N-substituted imines. 相似文献
26.
The title compound is a crucial intermediate in the biomimetic conversion of vincadifformine 1 into vincamine 7. Its configuration at C-16 is established by a combination of chemical and spectroscopic evidence. Iodine oxidation converts 14 into the bridged lactam 18, thus proving a β configuration for the hydroxy group at C-16. The same reaction applied to vindoline 19 gives 21 identical with one of the compounds obtained by microbiological transformation of 19. The 13C NMR spectra of derivatives 3 and 8 (obtained by oxidation of vincadifformine) show that oxidation proceeds with introduction of the substituent at C-16, with a β-configuration. The alcohol 3 however, posesses a different conformation due to strong hydrogen bonding with N-4. 相似文献
27.
The principle of tandem on-line continuous separation techniques as an alternative means of introducing samples into plasmas was applied to the development of a sensitive, selective and convenient method for the determination of arsenic by inductively coupled plasma atomic emission spectrometry (ICP-AES). Arsenic is continuously extracted as AsI3 into xylene from the sample dissolved in 0.1 M potassium iodide solution in 7.2 M hydrochloric acid. The xylene phase (containing the analyte) is continuously mixed on-line with NaBH4 in dimethylformamide and acetic acid solutions. Arsine is thus continuously generated directly from the organic phase and is separated in a gas—liquid separation device which prevents most of the xylene phase vapour from reaching the ICP. The system was optimized for the continuous extraction of AsI3, the direct generation of arsine from xylene and the final ICP determination of arsenic. Finally, the tandem on-line continuous separation ICP detection system was applied to the determination of arsenic in real samples (white metal, cast iron, cupro-nickel and orchard leaves standard materials). Very good agreement between the experimental results and the certified values was obtained. 相似文献
28.
Here we examined the fragmentation, on a quadrupole ion-trap mass spectrometer, of the protonated ions of a group of peptides containing one arginine and two different acidic amino acids, one being aspartic acid (Asp) or glutamic acid (Glu) and the other being cysteine sulfinic acid [C(SO2H)] or cysteine sulfonic acid [C(SO3H)]. Our results showed that, upon collisional activation, the cleavage of the peptide bond C-terminal to C(SO2H) is much more facile than that of the peptide bond C-terminal to Asp, Glu, or C(SO3H). There is no significant difference, however, in susceptibility to cleavage of peptide bonds that are C-terminal to Asp, Glu, and C(SO3H). To understand these experimental observations, we carried out B3LYP/6-31G* density functional theory calculations for a model cleavage reaction of GXG --> b2 + Gly, in which X is Asp, Glu, C(SO2H), or C(SO3H). Our calculation results showed that the cleavage reaction is thermodynamically more favorable when X = C(SO2H) than when X = Asp or C(SO3H). We attributed the less facile cleavage of the amide bond after Glu to that the formation of a six-membered ring b ion for Glu-bearing peptides is kinetically not as favorable as the formation of a five-membered ring b ion for peptides containing the other three acidic amino acids. The results from this study may provide useful tools for peptide sequencing. 相似文献
29.
We have calculated the positions of the avoided level crossings between (n+2)s, np states and nd, k Stark states in the Rydberg Stark states of the potassium atom with principal quantum number n comprised between 12 and 17. We have also studied the adiabatic electric field ionization thresholds for the above Rydberg
states. Both the ionization thresholds and the positions of avoided crossings have been calculated using the recently developed
Stark-adapted quantum defect orbital (SQDO) formalism. The presently reported values appear to be in very good agreement with
the available theoretical and experimental data. 相似文献
30.
Chuan-Pin Lee Ching-Yuan Liu Ming-Chee Wu Chun-Hua Pan Tsuey-Lin Tsai Hwa-Jou Wei Lee-Chung Men 《Journal of Radioanalytical and Nuclear Chemistry》2013,296(2):1119-1125
The sorption capacity of cesium (Cs) and selenium (Se) in crushed mudrock was demonstrated in this study through a 2-site Langmuir model. To employ such a numerical analysis, batch tests were applied in this study in synthetic seawater and groundwater with sorption/desorption kinetic experiments (time-dependent) and different concentrations (10?2–10?7 M). The 2-site sorption models, which correspond to two rate constants (λ 1 and λ 2), might be more adequate than 1-site sorption models in characterizing Cs and Se sorption/desorption according to the least square errors between the numerical analysis and the results of the batch tests. The fitting results showed that a 2-site Langmuir model is capable of appropriately describing Cs and Se sorption in mudrock. Consequently, the sorption capacity was calculated at about 0.06 mol/kg for Cs and at 0.015 mol/kg for Se. 相似文献